You need to load adobe flash. Chat is flash based.
http://get.adobe.com/flashplayer/?promoid=BUIGP
As Mokoa said, most apple users are SOL. Some version of the OS for some laptop and desktops can run flash. I am mactarded so I can't ell you more than that about Macs.
Will one of you Mac experts chime in on which versions of the OS or how to hack the OS to run flash? I have read that some versions can run it. Don't know which ones.
I am thinking the latest OS does. One of my lady friends here has gone mac and I know she was in chat right out of the box, unless some macgenius helped her out.